Double walled tube of high chromium steel

ABSTRACT

A DOUBLE WALL TUBE IS COMPOSED OF A TWICE COILED OR ROLLED COPPER-PLATED HIGH CHROMIUM STEEL STRIP ESSENTIALLY COMPOSED OF NOT HIGHER THAN 0.04% C, NOT HIGHER THAN 0.50% SI, NOT HIGHER THAN 0.50% MN, NOT HIGHER THAN 0.50% NI, 10.0-14.0% CR, NOT HIGHER THAN 0.50% AND NOT LOWER THAN C PERCENT X 10 OF TI, AND IRON BALANCE AND THE OUTER SURFACE OF THE TUBE BEING COATED WITH A LEADTIN COATING.

Aug. 27, 1974 TSUYOSHI OHAMA ETAI- 3,832,136

DOUBLE WALLED TUBE OF HIGH CHROMIUH STEEL Filed July 21. 1972 2Sheets-Sheet 1 FIG. I

C 0.I7 -0.073% Ni 0.I I 024 Si 10.3I -0.66% CI l0.86-I2.I0 MnO.30-0.55/o Ti-'0.20-0.68%

l I l l l Aug. 27, 1974 TSUYOSHI OHA MA ETAL 3,832,135

DOUBLE WALLED TUBE OF HIGH CHROMIUH STEEL 2 Sheetsi-Sheec 2 Filed July21. 1972 FIG. 2

O.25-O.45/o

Mn 3 0.28 -O.32 Ti O.36-O.44%

O A V [II SOFTENED ANNEALING STEEL H3OC X 3Mi n.HEATING AND .AVI

AIR COOLING ll I mozmmw A m5:

0 6 Il A Y AA 4 3 2 zo ozod m6 ISZMEZ ymzm; b 1 5515 3m;

United States Patent Oifice 3,832,136 Patented Aug. 27, 1974 3,832,136DOUBLE WALLED TUBE OF HIGH CHROMIUM STEEL Tsuyoshi Ohama, Ichikawa, andKonosuke Shinohara, Tokyo, Japan, assignors to Nisshin Steel Co. Ltd.,Tokyo, Japan Filed July 21, 1972, Ser. No. 273,771 Claims priority,application Japan, Aug. 4, 1971, 46/58,278 Int. Cl. B32b 15/00 U.S. Cl.29191 2 Claims ABSTRACT OF THE DISCLOSURE A double wall tube is composedof a twice coiled or rolled copper-plated high chromium steel stripessentially composed of not higher than 0.04% C, not higher than 0.50%Si, not higher than 0.50% Mn, not higher than 0.50% Ni, 10.0-14.0% Cr,not higher than 0.50% and not lower than C percent X of Ti, and ironbalance and the outer surface of the tube being coated with a leadtincoating.

'BACKGROUND' OF THE INVENTION Field of the Invention This inventionrelates to a double wall tube of high chromium steel and a process forproduction thereof. More particularly, this relates to a double walltube of high chromium steel which comprises a double wall tube of highchromium steel with a single phase ferrite similar to a plain carbonsteel in point of mechanical properties and a lead-tin coating on theoutside of the tube.

Description of the Prior Art Heretofore, a copper-plated double walltube has been used in place of a copper tube, as a feeding tube for thebrake fluid of car, a feeding tube for gas, a radiator tube for a heatexchanger and the like. This copper-plated double Wall tube may beproduced by cold-rolling a plain carbon steel to form a thin strip ofseveral hundreds microns in thickness, plating both surfaces with copperto form a copper layer of several microns thick, annealing the thinsteel strip thus plated, rolling the resulting tube material to producea tube and brazing the copper plating. The double wall tube thusobtained is preferable from the point of view of formability and cost.However, this double wall tube prepared from a plain carbon steel is notof sufiicient corrosion resistance under the recent severe corrosiveenvironment, and its durability is extremly short in certain severeenvironments. For the purpose of improving the durability, there havebeen recently employed a treatment of plating the outer surface withzinc or a rubber lining, but no satisfactory treatment has yet beenfound. I

In general, the inner surface of a tube is hardly subjected tocorrosion, but the outer surface of a tube is often subjected tocorrosion. For example, outer surface of a brake tube of car is exposedto outside atmosphere water and mud and therefore, corrosion proceedsfrom the plated layer and rubber lining through the copper coating tothe iron portion so that the life of the tube is not satisfactorilylong. Particularly, even when the outer surface of the tube is platedwith a lead-tin plating layer, the corrosion resistance against chlorineions is not effectively improved since the natural electrode potentialof the lead-tin alloy is nobler than that of iron and, therefore, theiron is subjected to corrosion in spite of the presence of the lead-tinplating layer on the outer surface of the tube.

There have been recently used clad materials of plain carbon steel andstainless steel plated With copper which have fairly high corrosionresistance and mechanical strength. Double Wall tubing using thestainless steel clad material solves the problems of corrosionresistance and durability to some extent, but is not yet satisfactory.The clad material is usually expensive due to complicated manufacturingprocesses.

It has been also recently proposed to manufacture a duplex roll tube byusing an inexpensive and corrosion proof stainless steel as the basematerial for the tube. However, conventional stainless steels aregenerally hard and do not have such easy workability as plain carbonsteels or stainless steel clad material and therefore, conventionalstainless steel can not be used for double wall tubes. In other words,when a conventional relatively hard stainless steel sheet or strip iswound up twice, shaped and brazed, the outer layer and inner layer ofthe double wall tube can not closely contact each other but form gapssince the stainless steel is hard and poor in bending workability andfurther has a buckling property. In an extreme case, the adjoining sheetsurfaces are separate from each other and each surface is separatelybrazed. Thus, any satisfactory double wall tube is not obtained.

In addition, a stainless steel itself is not easy to plate with abrazing metal such as copper, zinc, solder and the like.

SUMMARY OF THE INVENTION According to the present invention, there isprovided a double wall tube of high chromium steel produced by shaping,as by rolling or coiling a copper-plated high chromium steel stripessentially consisting of not higher than 0.04% C, not higher than 0.50%Si, not higher than 0.50% Mn, not higher than 0.50% Ni, 10.014.0% Cr,not higher than 0.50% and not lower than C percent X 10 of Ti, and ironbalance and then coating the outer surface of the tube with a lead-tincoating.

An object of this invention is to provide a double wall tube having highcorrosion resistance under severe conditions.

Another object of this invention is to provide a double wall tube freefrom transformation of structure such as hardening, embrittlement andloss of strength upon heating treatment in brazing.

A further object of this invention is to provide a double wall tubereadily adaptable to brazing.

Still another object of the present invention is to provide a doubleWall tube composed of a stainless steel of high chromium containing aspecified combination of components giving excellent workability.

Still further object of the present invention is to provide a method forproducing the double wall tube of high chromium.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing a relationbetween contents of C and Ti and transformation of steel structure whena chromium steel containing titanium is heated at 1150 C.; FIG. 2 is agraph showing a relation between chromium contents and mechanicalproperties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to solve theabove-mentioned problems in prior art, the present inventors have nowsucceeded in manufacturing an inexpensive and highly corrosion proof(even under severe conditions) double wall tube of high chromium steelby selecting the amounts of C, Si, Mn, Ni and Cr components at low levelas far as possible so as to lower the yield strength, tensile strengthand hardness to those of a plain carbon steel, adding titanium theretofor the purpose of preventing the steel from hardening and embrittlingafter brazing treatment and thereby producing a chromium seriesstainless steel having a single phase of ferrite, coating the resultingchromium series stainless stainless steel with copper, shaping thestainless steel thus copper-plated into a double wall tube and coatingthe outer surface of the resulting double wall tube with a lead-tincoating.

Contents of components in the high chromium steel used, as tube materialfor the double wall tube in the present invention are selected by thereason stated below.

Carbon is an important component affecting yield strength, tensilestrength, and hardness and moreover, is an effective element to formaustenite, but when the carbon content is higher, the steel becomes hardand formability for forming the double wall tube is lowered. Therefore,the carbon content should be not higher than 0.04%. At a carbon contenthigher than 0.04%, the heat treatment in brazing results in theformation of an austenite system and therefore a hard martensite is aptto form at normal temperature. This formation is not desirable from themechanical property point of view, and the titanium content should beincreased so as to prevent such formation. Therefore, the upper limit ofthe carbon content is set at 0.04%.

Silicon is an element capable of forming ferrite and suppressing theformation of martensite. When the silicon content is too high, the steelbecomes hard, as in the case of carbon content, so that the siliconcontent is kept as low as possible, and the upper limit is 0.50%.

Manganese is an element capable of forming austenite and is alsonecessary as a deoxidizer, but a higher content of manganese acceleratesthe formation of a martensite system. The upper limit is 0.50%.

Nickel is an important element for improving corrosion resistance as ischromium, but can form austenite and therefore, high nickel contentaccelerates the formation of a martensite system and furthermore,results in high cost. Therefore, the upper Ni limit is set at 0.50%.

Chormium is an element capable of forming ferrite and is important formaintaining corrosion resistance, but at a content of less than thecorrosion resistance is not sufficiently maintained while at a contentof higher than 14% the yield strength, tensile strength and hardness ofthe steel become so high that the bending workability is lowered.Therefore, the content range of chromium is set between 10.0 and 14.0%

Titanium is an element capable of forming ferrite and and also can forma stable compound with carbon and nitrogen. Therefore, titanium is anelement effective for complete ferrite formation. For formation ofcomplete ferrite the content of titanium should be not lower than 4 10times the content of carbon, but when the Ti content is higher than0.50%, the steel is hardened so that the optimum range of Ti content isnot higher than 0.50% and not lower than 10 times the C. percent. Inthis content range of Ti the sensitivity of intergranular corrosion canbe suppressed upon heat treatment in the brazing step.

As a brazing material for the steel plate used for a double wall tubeaccording to the present invention, there may be used a metal coatinglayer such as copper, zinc, solder and the like.

Particularly, copper plating is easily applicable to the steel materialused in the present invention. It is preferred to employ a copperplating layer as the brazing material for a usual usage.

When the copper-coated steel is annealed, the Vickers hardness becomesabout and this softness is almost the same as that of plain carbonsteel, and furthermore, when the steel is shaped into a double wall tubeand heated for hazing, there may be obtained a double wall tube of highchromium having high corrosion resistance and no void at the bondinglayer.

The corrosion resistance is further improved by applying a lead-tincoating to the outer surface of the copperplated double wall tube.

The double wall tube of high chromium itself according to the presentinvention show a good corrosion resistance even if the coating on theouter surface of tube is not provided, but a complete corrosionresistance against chlorine ion is not expected when the double walltube is used as a brake tube for a car and a freezing inhibitor such ascalcium chloride is scattered on a wet road in winter. Under such asevere condition as above, the corrosion resistance can be maintained byplating the outer surface of the copper-plated double wall tube withlead-tin.

When a copper-plated double wall tube is exposed to a corrosiveatmosphere containing chlorine ion, the copper is corroded with thechlorine ion and the corrosion rapidly reaches the steel portion. Thenatural electrode potential of the high chromium steel used in thepresent invention is more base than that of copper and there easilyoccurs pitting corrosion when exposed to a corrosive environment. On thecontrary, when a lead-tin coating is applied to the outer surface of thecopper-plated double wall tube, the direct attack of chlorine ion iseffected onto the lead-tin layer and this corrosion proceeds onlyslowly. Furthermore, natural electrode potential of lead-tin is morebase than that of the high chromium steel and therefore, as far as thelead-tin coating is present on the surface of the double wall tube, thehigh chromium steel portion nobler than lead-tin is not subjected tocorrosion. In case of plain carbon steel, the iron is more base thanlead-tin and therefore, the iron is still corroded in spite of thepresence of lead-tin on the surface and the lead-tin coating is not soeffective.

According to the present invention, the lead-tin coating on the copperplating of the outer surface of the copperplated double wall tube canimprove the corrosion resistance in a corrosive atmosphere containingchlorine ion, to a great extent.

The following example is given for illustrating the present invention,but should not be construed as restricting the present invention.

EXAMPLE A ferrite stainless steel containing Cr 11.26%, Ti 0.32%, Mn0.41%, Si 0.35%, and C 0.034% was coldrolled and cut to produce a steelstrip of 0.335 mm. thick and 27.5 mm. wide and both sides of theresulting high chromium stainless steel strip were activated afterpretreatment and copper-plated by using a plating solution containing200 g./l. of copper sulfate and 50 g./l.

of sulfuric acid to form copper coating of 5 microns The ratio of Ti -toC suitable for preventing transformathick on the whole surface of thesteel strip. The resulttion is higher than 10 as is clear from FIG.'1.ing copper coating was tightly adhered to the steel. (2) The abovementioned steel strips are plated with The resulting copper-plated steelstrip was continuously copper as a brazing material, shaped into adouble wall subjected to roll-processing to produce a double wall tube 5tube by roll-processing, heat-treated at a temperature of 4.76 mm. indiameter. Any burn of copper on the higher than melting point of thebrazing material to core spindle for determining the inside diameter ofthe finish brazing. For the purpose of con-firming formability tube wasnot observed at all. and properties of the double wall tube afterbrazing, The resulting copper-plated double wall tube was subthere arecompared mechanical properties of the steel jected to brazing at 1130 C.in a furnace and there was 10 strip itself and those of the steel stripafter heat treatment obtained a double wall tube free from voids. asshown in Table 2. Further, the resulting double wall tube was soaked inAs is clear from Table 2, the mechanical properties an alloy meltingbath of 75% lead and 25% tin at 300 of high chromium steel strip used inthis invention is C. for 10 seconds to produce a lead-tin coating of 8very close to those of a plain carbon steel strip and the microns thickon the outer surface of the copper-plated workability is excellent andfurther, the properties are double wall tube. not changed even afterheat treatment.

A copper-plated double wall tube coated with lead-tin (3) FIG. 2 showsthe effect of the composition of of the present invention was comparedwith a coppersteel upon mechanical properties before and after heatplated double wall tube without a lead-tin coating with treatment withrespect to the high chromium steel conrespect to a rust resistance test(refer to Table 3 infra). taining Ti used in the present invention. FIG.2 indicates The test was carried out by corrodkote test in accordthatthe chromium content should be lowered as far as ance with JIS-D-OZOI.The copper-plated double wall possible without losing minimum corrosionresistance. tube of high chromium steel without a lead-tin coating It ispreferable to lower Si, Mn and Ni contents as was better than a doublewall tube of plain carbon steel, far as possible. but whole surface ofthe former was covered with rust (4) The high chromium steel strip usedfor the douin 200 hours. On the contrary, no rust was formed on ble walltube of this invention has a far better corrosion the copper-plateddouble wall tube of high chromium resistance than plain carbon steelswhich have been used steel with a lead-tin coating at all in 500 hours.for conventional double wall tube and the corrosion re- As the result ofa continuous salt spraying test in acsistance is not deteriorated byheat treatment upon cordance with JISZ-2371, the copper-plated doublewall brazing. tube with a lead-tin coating formed no rust in 800 hoursResults of the rust resistance test are shown in Table 3. while thecopper-plated double wall tube without a lead- As stated above, thecopper-plated double wall tube tin coating formed rust on the wholesurface in 500 with a lead-tin coating has mechanical properties similarhours under the same atmosphere as above. This results to those of plaincarbon steel, and the mechanical propindicate that the copper-plateddouble wall tube of high erties are hardly changed by brazing. Further,the double chromium steel with a lead-tin coating according to the walltube according to the present invention has high present invention hasan excellent corrosion resistance corrosion resistance, formability andbending workeven in a corrosive atmosphere containing chlorine ion.ability.

The properties of the high chromium steel and the double wall tubeaccording to the present invention are compared with those ofconventional ones as shown below.

. TABLE 1 (1) Some examples of components of the high chromium steelstrip used in this invention and an example Element of components of aplain carbon steel strip are illustrated Type C 51 M11 P 5 Ni Cr Tl inTable 1. Plain carbonsteel 0.07 0.25 0.01 0.02

The high chromium steel used in the double wall tube gffi figfig g; hasrtic vention:

i lady low ,contents of carbon f and No.1 0.023 0.35 0.30 0.010 0.0000.20 11.28 0.30 the titanium content 1s lowered as far as possiblewithout No.2 0. 025 0.32 0.30 0. 018 0.010 0.21 11.00 0. 37 causingtransformation upon heat treatment in brazing. M20 0 31 0 28 M20 12'25TABLE 2 Steel strip after softening annealing 1,l30 C.X3 min. heatingand air cooling Yield Tensile Elon- Hard- Yield Tensile Elon-Hardstrength strength gation ness strength strength gation ness Type(kg/mm!) (kg/mm!) (percent) (HV) (kg/mm!) (kg/mm!) (percent) (HV) Plaincarbon steel 19 a4 49 88 20 30 45 90 High chromium steel in thisinvention:

No.1 17 40 42 10 37 40 103 No.2 17 40 41 100 10 88 39 10s No. 3 20 41 as112 19 39 as

